Refined Buoyancy Heat Engine for Glider Applications using Structures with Negative Thermal Expansion Coefficients
Abstract
Refined Buoyancy Heat Engine for Glider Applications using Structures with Negative Thermal Expansion Coefficients. Previous work has demonstrated the viability of shape memory alloys (SMA) actuators for oceanic operation. SMA actuators can exploit various opportunities for morphing autonomous undersea vehicles (AUVs). Previous research implemented SMA wires to create a SMA buoyancy heat engine to use the oceanic thermocline to drive an AUV. By coupling SMA elements to auxetic materials, the buoyancy heat engine can be improved, and SMAs can morph AUVs for drag reduction. In order to integrate SMAs and auxetics into AUVs, FEA must be used to model morphing structures under hydrostatic load. When the load on bi?stable structure elements exceeds a critical load, they snap?through to a second stable equilibrium position. The resulting large configuration change can be exploited for morphing applications. The focus in this task is to realize cavity volume change enabled by the use of bistable structural elements. The resulting increase in volume and buoyancy would cause the chamber to rise to the surface. Closer to the surface, snap?through can be used in the other direction to decrease volume change by activating SMAs working in the other direction. The configuration with lower volume and buoyancy would descend to the bottom again. When compressed in one direction, an auxetic lattice compresses in an orthogonal direction. Auxetic lattices generally possesses a greater stiffness to density ratio than the solid material of its construction. This is due to the geometric design of the cell (honeycomb, truss, chiral, etc). When mapped to a cylindrical body, this can enable a three dimensional volume change. The relative stiffness and volume change of a buoyancy heat engine could be improved by this three dimensional change. Increasing relative stiffness enables a glider to carry a heavier payload. Increasing volume change increases glider speed at a given payload and maximum payload. In addition to volume change, auxetic lattices can be leveraged to achieve a targeted shape when deformed. This is relevant to underwater vehicles insomuch as unloaded glider sections can be designed to morph into useful shapes at depth. One such example of this is to design a glider section of circular cross section that morphs into an ellipse when loaded. Elliptical airfoils are well studied shapes for producing lift. An auxetic cylinder with a spatial stiffness gradient can be designed to approximate an ellipse when loaded by an actuator. This design would incorporate the elasticity of the lattice and a low strain skin to create a control surface or reduce drag. Unlike a buoyancy engine, a morphing auxetic control surface does not need to work against the hydrostatic pressure.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Nov 23, 2016
- Source ID
- N000141612680
Entities
People
- Farhan Gandhi
Organizations
- Office of Naval Research
- Rensselaer Polytechnic Institute
- United States Navy